Mineral Precipitation-induced Pore Clogging and its Effect on Transport Parameters in Diffusion-controlled Porous Media
2 Water and Environment division, French Geological Survey (BRGM), Orléans, France
3 Department of Geosciences, Johannes Gutenberg University, Mainz, Germany
4 French Agency for Nuclear Wastes Management (ANDRA), Châtenay-Malabry, France
P 4.7 in Endlager und Untertagedeponien
In geochemically perturbed systems due to porewater disequilibrium mineral precipitation/ dissolution might be induced which could possibly change the transport properties as porosity and pore diffusion coefficient. These reactions might alter the sealing capabilities of the rock by complete pore-scale precipitation (cementation) of the system or by opening new migration pathways through mineral dissolution. In actual 1D continuum reactive transport codes the coupling of transport and porosity is generally accomplished through the empirical Archie’s law. Experimental data on its general applicability for systems changing the porosity under well controlled conditions to constrain model input parameters to a maximum possible are rarely documented. In this study, celestite (SrSO4) was precipitated in the pore space of a compacted sand column under diffusion-controlled conditions and the effect on the fluid migration properties investigated by means of three complementary experimental approaches: (1) tritiated water (HTO) tracer through diffusion, (2) computed micro-tomography (µ-CT) imaging and (3) post-mortem analysis of the precipitate (selective dissolution, SEM/EDX).
The through-diffusion experiments reached steady state after 15 days, at which point celestite precipitation ceased and the non-reactive HTO flux became constant. The pore space in the precipitation zone remained fully connected under the 6µm µ-CT spatial resolution with 25% porosity reduction in the approx. 0.35 mm thick dense precipitation zone. The porosity and transport parameters prior to pore-scale precipitation were in good agreement with a porosity of 0.42 ± 0.09 (HTO) and 0.40 ± 0.03 (µ-CT), as was the mass of SrSO4 precipitate estimated by µ-CT with 25 ± 5 mg and selective dissolution 21.7 ± 0.4 mg, respectively. However, using the experimentally derived data as input parameters, the 1D continuum reactive transport model that assumed the direct linkage of porosity to the effective diffusivity via one cementation factor valid over the whole porosity variation range of the system investigated was not able to accurately reproduce both the celestite precipitation front and the remaining connected porosity.
The 1D continuous model either underestimated the remaining connected porosity in the precipitation zone, or overestimated the amount of precipitate to provide a best fit of the experimental data. These findings support the need to implement a modified, extended Archie’s law to the reactive transport model and show that pore-scale precipitation transforms a system (following Archie’s simple power law with only micropores present) towards a system similar to clays with micro- and nanoporosity.
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